CVE-2026-1425: 
Linux Debian vulnerability analysis and mitigation

CVE-2026-1425 is a stack-based buffer overflow vulnerability in pymumu SmartDNS, a popular local DNS server application. The flaw exists in the _dns_decode_rr_head and _dns_decode_SVCB_HTTPS functions within src/dns.c, specifically in the SVCB/HTTPS record parser component. It affects SmartDNS versions up to and including 47.1 and can be triggered remotely without authentication. The vulnerability carries a CVSS v3.1 base score of 5.6 (Medium) and a CVSS v4.0 base score of 6.3 (Medium), with high attack complexity noted (VulDB, Red Hat CVE).

The root cause is improper bounds checking (CWE-119/CWE-121) when parsing DNS SVCB and HTTPS record types. In _dns_decode_rr_head, the resource record length (rr_len) was not validated against the remaining buffer length before use, and similarly in _dns_decode_SVCB_HTTPS, the HTTPS data length was not checked against the available buffer. An attacker can craft a malicious DNS response containing an oversized SVCB/HTTPS record that causes a stack-based buffer overflow when SmartDNS processes it. The fix, committed as 2d57c4b4e1add9b4537aeb403f794a084727e1c8, adds explicit bounds checks in both functions to return an error if the record length exceeds the remaining buffer (GitHub Commit).

Successful exploitation could result in low-level impacts to confidentiality, integrity, and availability of the SmartDNS process, as reflected in the CVSS scoring. In a worst-case scenario, a stack-based buffer overflow could be leveraged to achieve arbitrary code execution in the context of the SmartDNS service, potentially allowing an attacker to manipulate DNS responses, intercept or redirect network traffic, or crash the DNS service causing denial of service for all clients relying on it (VulDB, Red Hat CVE).

No public proof-of-concept exploit code or in-the-wild exploitation has been reported for CVE-2026-1425. The EPSS score is approximately 0.039%, indicating a low probability of exploitation in the near term. The vulnerability is rated as high complexity, and VulDB notes that exploitability is considered difficult. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. It is detectable via Tenable Nessus plugin 296605 (Tenable, VulDB).

1. Reconnaissance: Identify hosts running SmartDNS versions up to 47.1 that are accessible on the network, typically listening on UDP/TCP port 53.
2. Craft malicious DNS response: Prepare a specially crafted DNS response packet containing a SVCB or HTTPS record type where the rr_len field specifies a length larger than the actual remaining data in the packet buffer.
3. Trigger parsing: Position the attacker as a man-in-the-middle or operate a malicious upstream DNS server that SmartDNS queries. When SmartDNS forwards a query and receives the crafted response, the vulnerable _dns_decode_rr_head or _dns_decode_SVCB_HTTPS function processes the oversized record.
4. Overflow the stack buffer: The missing bounds check causes SmartDNS to read beyond the allocated stack buffer, overwriting adjacent stack memory including return addresses or control data.
5. Achieve objective: Depending on memory layout and exploit sophistication, this could result in a crash (denial of service) or, with further exploitation techniques, arbitrary code execution in the SmartDNS process context (GitHub Commit, VulDB).

• Network: Unexpected or malformed DNS SVCB/HTTPS record responses received by the SmartDNS host from upstream DNS servers; unusual DNS traffic patterns or responses from unknown upstream resolvers.
• Logs: SmartDNS debug logs containing messages such as rr len exceeds remaining buffer. or https data length exceeds buffer. (added by the patch), which may indicate attempted exploitation even on patched systems; repeated crashes or restarts of the SmartDNS process.
• Process: Unexpected termination or crash of the SmartDNS daemon (smartdns process); core dump files generated in the SmartDNS working directory following abnormal termination.

The vendor has addressed this vulnerability by applying patch commit 2d57c4b4e1add9b4537aeb403f794a084727e1c8 to the SmartDNS repository, which adds bounds checks in both _dns_decode_rr_head and _dns_decode_SVCB_HTTPS. Users should update to a version of SmartDNS that includes this patch (released after version 47.1; the latest release as of the repository is 48.1). As a temporary workaround, restricting SmartDNS to only query trusted upstream DNS servers reduces the attack surface, since exploitation requires a malicious DNS response to be delivered to the SmartDNS instance (GitHub Commit, SmartDNS Repo).

The vulnerability was noted by VulDB upon disclosure and picked up by automated vulnerability tracking feeds including Vulners, CIRCL, and INCIBE-CERT. A brief mention appeared on Mastodon via the VulDB account and on Bluesky. No significant vendor statements beyond the patch commit or notable researcher commentary have been identified (VulDB).